Recently, the integration of device components in a computer is getting denser and, therefore, the requirement to the components of the computer is that they have to be lighter, thinner, shorter and less power consumption. Comparing to the other display devices, such as cathode ray tube (CRT), the LCD is a small-sized device and has a low power consumption. In addiction to a liquid crystal screen, the LCD requires a light source because the liquid crystal is not a self-luminous substance.
Currently, lamps, such as CCLF (Cold Cathode Fluorescent Lamp) and HCFL (Hot Cathode Fluorescent Lamp), have been used as a light source for LCD. A back light unit which contains lamps as a major light source for LCD can be divided into two types, i.e., the direct-lighting type and the edge lighting type, according to the location of the lamp source. In the direct-lighting type, lamp light is uniformly distributed by a diffusing sheet and then emitted into the liquid crystal panel. On the other hand, lamp light of the edge lighting type transmits through a light guiding plate to form a planar light source
FIG. 1 illustrates a conventional direct-lighting type back light unit. Lamps 10 are positioned in and surrounded by a lamp reflector 20 for reflecting the light emitted from the lamps 10 upward. An optical composition board constituted of a diffusing plate 50, a lower prism plate 60 and a upper prism plate 70 in sequence from the bottom to the top is mounted to the lamp reflector 20 for the transmission of the light from the lamps 10.
The diffusing plate 50 diffuses and uniformly distributes incident light to project uniform light intensity onto a liquid crystal panel (not shown). The lower and upper prism plates 60 and 70 convert the light travel path and are composed of a sequence of troughs in the shape of triangles or hemispheres. To protect the structured shape of the lower and upper prism plates 60 and 70, a protecting plate 80 is disposed on the upper prism plate 70. The liquid crystal panel is disposed on the protecting plate 80.
In the back light units as described above, the light emitted from the lamps 10 is reflected by the reflector 20, and then images on the display screen are formed by the light passing through the diffusing plate 50. The light transmitted through the diffusing plate 50 is directed toward perpendicularly to the liquid crystal panel with the light travel path converted to a selected angle by passing through the lower and the upper prism plates 60 and 70.
However, to enhance uniformity of light for the back light unit, there must be a certain space between the diffusing plate 50 and the lamps 10 for eliminating potential dark strips (as shown in FIG. 2) caused by the reduced intensity of light between adjacent lamps 10. Heretofore, a layer of particular ink 51 is printed on the diffusing plate 50 (as shown in FIG. 3) in order to further diffuse the light. The method solves the problems of dark strips and the likelihood of visual observation of the lamps by bare eyes. However, it suffers from increased thickness and decreased luminance due to the space exiting between the diffusing plate and light source. In addition, the ink itself absorbs energy of the light. Further, the ink may get aged, making back light unit become yellowish.
In order to overcome the defects mentioned above while minimizing the back light unit, there is a need for a back light unit that solves the above problems, such as poor uniformity, direct observation of the lamps, low luminance, increased thickness and the yellowish issue.